Spinal fixation systems may be used in surgery to align, adjust and/or fix portions of the spinal column, i.e., vertebrae, in a desired spatial relationship relative to each other. Many spinal fixation systems employ a spinal rod for supporting the spine and for properly positioning components of the spine for various treatment purposes. Vertebral anchors, comprising pins, bolts, screws, and hooks, engage the vertebrae and connect the supporting rod to different vertebrae.
The length of the cylindrical rod depends on the size and number of vertebrae to be held in a desired spatial relationship relative to each other by the apparatus. The rods may be bent, as desired, to conform to a desired curvature of the spinal column in one or more of the anatomic planes. The size of the spinal rod depends on the region of the spine where the spinal fixation system is used. For example, in the cervical region of the spine, where the vertebrae tend to be smaller, a relatively smaller spinal rod is used, which is positioned close to the center of the spine. In the thoracic region, where heavier loads are experienced and the vertebrae tend to be larger, a rod having a larger diameter is used. The cervico-thoracic junction of the spine is typically fused using rods of two different diameters to accommodate anatomical differences between the cervical and thoracic spine regions. To accommodate a system including spinal rods having different sizes and configurations, a rod connector may be used to join a first rod and a second rod. The rod connector may be a side-by-side connector, where the ends of the two rods are placed side-by-side and connected using a connector that spans the two ends, or an axial connector, which aligns the axes of the two rods and connects the ends of the rods together along the axial direction.
Prior rod-to-rod connectors for the cervico-thoracic junction are static and include fixed, parallel bore holes for receiving spinal rods that are spaced an equal distance from the spinal column, i.e., having axes that are aligned in the sagittal plane of the patient when the connector is implanted. However, the spinal support rods that are joined using a rod connector are frequently oriented at various angles and positions due to the anatomical structure of the patient, the physiological problem being treated, and the preference of the physician. Because the two rods are configured to accommodate variations in the spinal column, the portions of the rods that are connected by the connector may not be parallel to each other, or spaced the same distance from the spinal column. The discrepancy must be fixed by bending the rods in other regions, such that the portions that insert in the bore holes are aligned and parallel with each other. However, bending tends to weaken the rods and can be an imprecise method for ensuring that the rods fit in the connector.